This invention relates to a band narrowing laser configured so as to correct wavefront distortions in laser beams generated in a band narrowing module.
The use of excimer lasers as light sources in steppers used in semiconductor device manufacture is now drawing considerable attention. There are a number of reasons for this. Excimer lasers have short wavelengths, making it possible to extend the limit of light exposure to 0.35 xcexcm and below. With the same resolution, the depth of field is deeper as compared to the g lines or i lines of mercury lamps. The numerical aperture (NA) maybe small. The exposure areas can be made large. And great power can be obtained. These are some of the outstanding advantages that can be expected.
However, when this excimer laser is used as the light source in a semiconductor exposure apparatus, synthetic quartz material is the only lens material that can be used in fabricating the optical system at excimer laser wavelengths (the wavelength of a KrF excimer laser being 248 nm and that of an ArF excimer laser being 193 nm) (it being difficult to machine CaF2). With simple synthetic quartz materials, however, chromatic aberration functions cannot be imparted.
In the case of naturally oscillating light from a KrF excimer laser, for example, the spectral line width is a broad 300 pm wherewith, if unchanged, the chromatic aberration of the exposure apparatus lens cannot be disregarded, and it is not possible to obtain sufficient resolution in the exposure results.
That being so, when an excimer laser is used as the light source in a semiconductor exposure apparatus, the band of the laser beam is narrowed by deploying a wavelength selection element such as an etalon or grating and prism, etc., in the laser resonator.
Inside the light resonator, however, due to various causes, the laser beam wavefront will come to exhibit divergence (broadening) and curvature.
When a slit is made inside the resonator, for example, the light will become a spherical wave after passing through the slit, due to the diffraction produced by the slit.
There are also cases where the wavefront is distorted by the aberration of the optical element itself that is disposed in the resonator. With a transmissive type of optical element such as a prism expander-used as a band narrowing element, for example, due to such facts as (a) that the internal refractive index distribution is not uniformly perfect and (b) that the polished surfaces of the prism are distorted, the wavefront of a laser beam that has passed through such an optical element will exhibit either convex or concave curvature.
Also, when a laser beam having a wavefront exhibiting such curvature strikes a grating of flat shape, the wavelength selectivity of the grating is diminished. That is, when the incident wavefront of a laser beam on a grating has curvature, the laser beam will strike each of the grooves in the grating at a different angle, wherefore the wavelength selectivity of the grating diminishes, and the spectral line width of the, band-narrowed laser beam broadens.
Thereupon, in U.S. Pat. No. 5,095,492, the difficulties noted above are dealt with by bending the grating itself so as to coincide with the wavefront of the laser beam incident on the grating.
More specifically, a band narrowing excimer laser in this prior art, as diagrammed in FIG. 24 has a front mirror 100, a laser chamber 101, an aperture 102, a beam expander 103, a mirror 104, and a grating 105. Therein, moreover, the grating 105 is bent according to the curvature of the wavefront incident on the grating 105 by a curvature generating apparatus such as that diagrammed in FIG. 25.
The curvature generating apparatus diagrammed in FIG. 25 is configured so that the grating 105 is supported at both ends by mounts 107 via balls 106. Therein, the mounts 107 are linked to a pressure plate 109 through springs 108, with one end of a bolt 110 screwed into the pressure plate 109, and the other end thereof screwed into a nut 111 joined to the center of the back side of the grating 105. By turning the bolt 110 and thereby pulling the center of the grating 105 toward the pressure plate, concave curvatures are produced in the grating 105, as diagrammed in FIG. 26.
Also, with this prior art, appropriate grating tension is preset according to the spectral line width of the laser beam, and, based on that setting relationship, a motor 113 is driven and controlled to drive and turn the bolt so that the tension on the grating becomes a set tension corresponding to a value detected by a spectral line width detection sensor 112.
With the prior art described in the foregoing, the grating itself is bent to impart curvature thereto according to a detected laser beam spectral line width.
However, in the grating, in order to control the laser oscillating wavelength to the desired wavelength, it is necessary to control the angle at which the light is incident on the grating. For this purpose, the grating is provided with a turning mechanism such as a turning stage, so that the grating can be turned as indicated by the arrow J in FIG. 24 by the turning mechanism.
Therefore, for the grating in the conventional apparatus described above, in addition to the curvature generating mechanism described for producing curvatures in the grating, it is also necessary to install a turning mechanism to turn the grating. Such a mechanism is not only complex, bulky, and impractical, but the vibration produced when the grating is turned is transmitted to the curvature generating mechanism and that vibration can result in variation in the spectral line width.
With a band narrowing excimer laser, moreover, a large grating is required to get the spectral line width down to or below 1 pm, and it is all but impossible to fabricate a large grating, even if it is flat, that exhibits uniform groove intervals without distortion. What is worse, with the prior art described above, since curvature is to be imparted to such a flat grating, groove interval unevenness and distortion are exacerbated, making it impossible to obtain suitable wavelength selection characteristics.
Furthermore, with the prior art described above, the grating itself is bent to correct the wavefront aberration of the light incident on the grating, giving rise to a problem in that, when there is wavefront aberration in the grating itself, it is not possible to correct that aberration.
An object of the present invention, which was invented in view of the situation described in the foregoing, is to provide a band narrowing laser wherewith laser beams can be obtained which exhibit narrow and stabilized spectral band width.
In this invention, a band narrowing laser having a band narrowing module for band-narrowing and outputting a laser beam generated from a laser medium by a band narrowing element, in which the band narrowing module comprises wavefront correction means for correcting the wavefront of an incident laser beam and outputting that laser beam.
In other words, according to the present invention, wavefront correction means for correcting the wavefront of a laser beam, and outputting that laser beam, are provided in a laser narrowing module.
For this reason, as based on the present invention, laser beams having a very narrow spectrum can be efficiently and stably output. With the present invention, furthermore, there is no bending mechanism for correcting the wavefront aberration of a grating, wherefore the spectral line width can be stabilized when controlling the wavelength.